Hydrophones have been used for many years for detection and location of ships and submarines and other underwater targets. Hydrophones are also used for sensing sonic waves in underwater geophysical exploration. A hydrophone receives sonic waves from underwater sources such as noise generated by a submarine or sonic waves reflected from an object. A single hydrophone may be used for "listening" for the presence of a surface object or an underwater object or for sensing small pressure waves.
The hydrophone is a dynamic sensor or transducer which converts the pressure waves in the water to electrical signals. An associated circuit with filtering and amplification functions conditions the signal for transmission via a cable to an electronics unit where the information is evaluated for data collection or for tactical purposes. Low frequency dynamic sensors are needed for high resolution pressure change measurement at frequencies much less than 1 Hz.
A useful bit of information for the evaluation of hydrophone signals is the depth of the hydrophone. A variable reluctance type dynamic transducer and the associated circuit has been proposed to provide a measure of static pressure which correlates to depth as well as dynamic pressure changes, but this requires a compromise in design so that neither the static pressure or dynamic pressure measuring functions are optimized.
It is already known to incorporate more than one sensor element and specifically more than one type of sensor in an underwater instrument. The Seaver U.S. Pat. No. 4,749,254 discloses an oceanographic instrument having an optical sensor system for measuring temperature, pressure and the index of refraction. The Savit U.S. Pat. No. 4,648,082 discloses an optical pressure gradient sensor for sensing the direction and magnitude of an acoustic wave propagating through a fluid. The Junod et al U.S. Pat. No. 4,315,324 discloses a hydrophone having two independent oscillators for a pressure sensor. However, the prior art does not appear to disclose separate sensors for measuring pressure wave events and depth. Generally the prior art transducers are operative at acoustic frequencies above 1 Hz and do not provide a means for measuring pressure waves which have a frequency below 1 Hz.